of the Earth's Crust. 425 



determining the thickness of the crust of the earth, provided we 

 possessed the requisite knowledge; but I never dreamed that 

 any mathematician coukl seriously imagine from my ])aper that 

 I attached any value of a positive character to such a calculation. 

 It is difficult to conduct a controversy between Calcutta and 

 Dublin; but I am sure your readers will agree with me^ that if 

 Archdeacon Pratt and I could communicate with each other in 

 a right line, we should both have our knowledge of the interior 

 of the earth considerably improved. I confess I do not see how 

 we arc to know more than we do about it, until some such tun- 

 nel shall have been made, pending which I shall remain a sceptic. 

 I am, Gentlemen, 



Your obedient Servant, 

 Trinitv College, Dublin, Samuel Haughton. 



October 22, 1859. 



Posiscripium. — Since writing the foregoing, I have read with 

 some surprise Archdeacon Pratt's further paper "On the Thick- 

 ness of the Earth's Crust," in the Philosophical Magazine for 

 the present month ; on the first part of which I beg leave to 

 make the following reraai'ks. 



He has omitted, in his consideration of the equilibrium of the 

 Ilimalayah Mountain, the principle of the arch, which is the 

 most important feature of the problem. 



I now restore the arch as follows, and show that a thickness 

 of 0-272 of a mile at Dehra is abundantly sufficient to support 

 the Ilimalayah Mountain. 



The minimum thickness of an arch composed of smooth vous- 

 soirs, is found from the condition that the curve of pressure 

 shall touch the Extrados at the crown, and tlie Intrados at the 

 skewback of the arch. 



To apply this principle, I draw a tangent at the summit of the 

 Ilimalayah to meet the radius through the centre of gravity 

 (making an angle 1° 13' 20", as supposed by Archdeacon 

 Pratt, for an approximation), and, from the point of intersection, 

 let fall a perpendicular on the radius drawn through Dehra. 

 The foot of this perpendicular lies 0-274 mile inside the surface 

 of the ground at Dehra. This depth is the minimum thickness 

 of the crust at this point. 



It is to be observed that this result is entirely irrespective of 

 cohesion or friction, wliich cannot come into operation until the 

 elementary condition of equilibrium of the arch fails. 



Hut it may be said that the crushing force due to the weight 

 of the mountain, will overcome tlie tenacity of the igneous rock 

 of which thcsiunmitof the Ilimalayah is conijJOsed.This objec- 

 tion 1 dispose of as follows. 



